1. Field of the Invention
The present invention relates to an optical film by which the optical path of the light incident on one of side surfaces of a liquid-crystal display device is changed to a viewing direction efficiently, and with which it is possible to form a transmission type or reflection-transmission double type liquid-crystal display device which is small in thickness, which is light in weight, which is excellent in brightness and uniformity of brightness and display of which is easy to view.
The present application is based on Japanese Patent Applications No. 2000-4241, 2000-21309 and 2000-85718, which are incorporated herein by reference.
2. Description of the Related Art
Greater reduction in thickness, size and weight of transmission type liquid-crystal display devices has been demanded for purposes of suppression of increase in weight which is accompanied by increase in size of television and personal computer display screens, reduction in size and weight of portable personal computers and portable telephone sets, etc. In the meanwhile, it is difficult to reduce thickness, size and weight of a transmission type liquid-crystal display device provided with a back-lighting system using a background-art bottom type or side-lighting type light pipe. Incidentally, the bottom-type back-lighting system generally has a thickness of not smaller than 4 mm because an illuminator, a light diffusing plate and a reflector are disposed just under a liquid-crystal display panel. Even the side-lighting type light pipe has a thickness of not smaller than 1 mm under the necessity of light transmission. When a light diffusing plate, a reflector, a prism sheet, etc. are disposed on the side-lighting type light pipe, the total thickness generally reaches a value of not smaller than 3 mm.
A liquid-crystal display device in which a half-transmission type reflector is disposed between the aforementioned transmission type liquid-crystal display panel and a back-lighting system is heretofore known as a reflection-transmission double type liquid-crystal display device which can be viewed in a reflection mode by using external light. The half-transmission type reflector is disposed in order to make viewing in a reflection mode possible. If there is no half-transmission type reflector, viewing in a reflection mode by using external light is so dark that the liquid-crystal display device substantially hardly functions as a reflection type liquid-crystal display device. The addition of the half-transmission type reflector, however, makes the volume and weight of the liquid-crystal display device larger. Moreover, light is diverged into transmitted light and reflected light by the half-transmission type reflector. There is therefore a problem that not only viewing in a transmission mode but also viewing in a reflection mode becomes dark so that brightness in a reflection mode is inferior to that of a reflection exclusive type liquid-crystal display device using a high-reflectance reflection layer.
An object of the present invention is to develop an optical film by which the optical path of the light incident on one of side surfaces of a liquid-crystal display device is changed to a viewing direction efficiently, and with which it is possible to form a transmission type or reflection-transmission double type liquid-crystal display device which is small in thickness, which is light in weight, and display of which is bright and easy to view.
According to a first aspect of the present invention, there is provided an optical film comprising: a transparent film; an adhesive layer provided on one surface of the transparent film, the adhesive layer having a refractive index different by 0.1 or less from a refractive index of a layer of the one surface of the transparent film; and a repetitive prismatic structure provided on the other surface of the transparent film, the repetitive prismatic structure having optical path changing slopes aligned in a substantially constant direction at an inclination angle in a range of from 35 to 48 degrees with respect to a plane of the transparent film.
According to a second aspect of the present invention, there is also provided an optical film comprising: a transparent film having an average in-plane retardation of not larger than 30 nm; an adhesive layer provided on one surface of the transparent film, the adhesive layer having a refractive index different by 0.12 or less from a refractive index of a layer of the one surface of the transparent film; and a repetitive prismatic structure provided on the other surface of the transparent film, the repetitive prismatic structure having optical path changing slopes aligned in a substantially constant direction at an inclination angle in a range of from 35 to 48 degrees with respect to a plane of the transparent film.
According to a third aspect of the present invention, there is also provided an optical film comprising: a transparent film having a refractive index of not lower than 1.49; transparent adhesive means provided on one surface of the transparent film, the transparent adhesive means having a refractive index of not lower than 1.49; and a repetitive prismatic structure provided on the other surface of the transparent film, the repetitive prismatic structure of having optical path changing slopes aligned in a substantially constant direction at an inclination angle in a range of from 35 to 48 degrees with respect to a plane of the transparent film. There is further provided a liquid-crystal display using the optical film.
The optical film according to the present invention is disposed along the viewing surface of a liquid-crystal display panel having an illuminator on one of side surfaces of the panel. Hence, the optical path of the light incident on the side surface or the transmission light of the incident light is changed efficiently to the viewing direction of the liquid-crystal display panel by optical path changing slopes disposed on the optical film. Hence, the light can be utilized for liquid-crystal display in a transmission mode. Hence, it is possible to form a transmission type liquid-crystal display device which is small in thickness and light in weight, which is excellent in brightness and uniformity of brightness, which is low in display unevenness and which is excellent in display quality. Moreover, because flat surface portions are disposed between the optical path changing slopes in the optical film, external light can be made to enter efficiently through the flat surface portions. Hence, when the entering external light is reflected by the reflection layer, the external light can be utilized for liquid-crystal display in a reflection mode. A reflection mode system can be formed as well as the aforementioned transmission mode system. Hence, it is possible to form a transmission-reflection double type liquid-crystal display device which is small in thickness and light in weight, which is excellent in brightness and uniformity of brightness, which is low in display unevenness and which is excellent in display quality.
The aforementioned effect is produced by an optical path control type optical film which mainly uses slope reflection to control the optical path of light. That is, the light incident on one of side surfaces of the liquid-crystal display panel or the transmission light of the incident light is reflected by optical path changing slopes so that the optical path of the light can be changed with good directivity. Hence, good visibility in a transmission mode can be achieved. Moreover, flat surfaces can be disposed easily between the optical path changing slopes. Hence, external light is transmitted through the flat surfaces so that entering of external light can be ensured sufficient. Hence, good visibility in a reflection mode can be also achieved. In a method of scatter reflection by a roughened surface of a scattering sheet 6 shown in FIG. 14, or the like, it is difficult to achieve the aforementioned effect. Incidentally, JP-A-5-158033 discloses a reflection type liquid-crystal display device in which illumination light is made incident on one of side surfaces of a liquid-crystal display panel and totally reflected by a visual side cell substrate and in which the reflected light is scattered by a roughened surface type reflector so that the scattered light is utilized for display.
In the aforementioned case, however, light allowed to be utilized for display is that which exits from the panel due to coming contrary to the total reflection condition by scattering. Generally, scattered light exhibits a normal distribution having a direction of regular reflection as a peak, in Extended Abstracts (the 20th Liquid-Crystal Discussion Lecture) 3 G510, Tohoku University; Uchida et al. Hence, the aforementioned display light is the light hardly utilized efficiently for display and greatly inclined with respect to a frontal (vertical) direction. Hence, the display becomes dark in the frontal direction. Nevertheless, intensifying scattering by the roughened surface type reflector is unfavorable for display in consideration of viewing in a reflection mode because the quantity of light in the frontal direction in the reflection mode is reduced (SID 96 DIGEST pp. 149-152). In the roughened surface scatter reflection method, it is, therefore, difficult to obtain scattering intensity favorable to the two modes because scattering intensity required of the transmission mode is antinomic to scattering intensity required of the reflection mode.
On the other hand, according to the present invention, the optical path control type optical film, which uses slope reflection to control the optical path of light, mainly utilizes light exhibiting a peak in a direction of regular reflection and controls the optical path of the reflected light. Hence, directivity, especially frontal directivity, favorable for display can be provided easily. Hence, a bright transmission mode can be achieved. Also in a reflection mode, flat portions of the optical film except the optical path changing slopes can be utilized, and efficient entrance, reflection and transmission of external light can be ensured. Hence, the state of light can be balanced easily so as to be favorable to both reflection and transmission modes. Further, according to the present invention, the optical film is designed to be able to be bonded to a glass substrate, or the like, of a liquid-crystal cell through adhesive means having a large refractive index. Hence, as indicated by the arrows in FIG. 7, the total reflection on the bonding interfaces can be reduced in the optical film to thereby make it possible to form a transmission type or reflection-transmission double type liquid-crystal display device which is excellent in brightness and uniformity of brightness, which is low in display unevenness and which is excellent in display quality. If the total reflection were great in the optical film, the quantity of light which is incident on one of side surfaces of the cell and which is transmitted through the cell from the side surface so as to enter the optical film would be reduced. Particularly, transmission light which is more approximately parallel with the cell substrate as indicated by the arrow in FIG. 10 would cause total reflection more easily. That is, light which is transmitted to a position farther from the incidence side surface would cause total reflection more easily. As a result, brightness in a position far from the incidence side surface would be reduced to increase variation in brightness. Hence, display quality of the display device would be lowered.
Features and advantages of the invention will be evident from the following detailed description of the preferred embodiments described in conjunction with the attached drawings.